This invention relates to ophthalmological surgery techniques for providing refractive corrections to the eye of a patient. More particularly, this invention relates to a method and apparatus for determining the astigmatism axis of a patient's eye in association with a surgical procedure.
Ophthalmological surgery techniques are known for correcting the refractive characteristics of a patient's eye to improve vision. One such technique is radial keratotomy, a surgical procedure which involves altering the contour of the cornea by making radial incisions through the corneal surface in a preselected peripherally distributed pattern. Another such technique known as photorefractive keratectomy (PRK) employs an ultraviolet laser to generate a beam capable of producing ablative photodecomposition of corneal tissue, the irradiated flux density and exposure time of the cornea to the ultraviolet laser radiation being so controlled as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea. Ultraviolet laser based systems and methods are disclosed in the following U.S. patents and patent applications, the disclosures of which are hereby incorporated by reference: U.S. Pat. No. 4,665,913 issued May 19, 1987 for "METHOD FOR OPHTHALMOLOGICAL SURGERY"; U.S. Pat. No. 4,669,466 issued Jun. 2, 1987 for "METHOD AND APPARATUS FOR ANALYSIS AND CORRECTION OF ABNORMAL REFRACTIVE ERRORS OF THE EYE"; U.S. Pat. No. 4,732,148 issued Mar. 22, 1988 for "METHOD FOR PERFORMING OPHTHALMIC LASER SURGERY"; U.S. Pat. No. 4,770,172 issued Sep. 13, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. Pat. No. 4,773,414 issued Sep. 27, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. patent application Ser. No. 109,812 filed Oct. 16, 1987 for "LASER SURGERY METHOD AND APPARATUS"; and U.S. Pat. No. 5,163,934 issued Nov. 17, 1992 for "PHOTOREFRACTIVE KERATECTOMY".
In the above-cited U.S. Pat. No. 4,665,913 several different techniques are described which are designed to effect corrections for specific types of optical errors in the eye. For example, a myopic condition, is corrected by laser sculpting the corneal surface to flatten the curvature. In addition, an astigmatic condition, which is typically caused by a cylindrical component of curvature departing from the otherwise generally spherical curvature of the surface of the cornea, is corrected by effecting cylindrical ablation about the axis of cylindrical curvature of the eye. Other optical errors can be corrected in a similar fashion.
Prior to performing a surgical procedure to effect a refractive correction for astigmatism, it is essential to determine the astigmatic axis of the patient's eye. A patient having astigmatism with spherical correction along one refractive meridian will see point sources of images smeared along a line. Consequently, a line which is aligned with the smear in an astigmatic eye's image will still appear as a line, but will become blurred when it is rotated. Patterns such as radial lines have been used in the past to approximately locate a patient's refractive cylinder axis. Another technique for making such a determination is disclosed in U.S. Pat. No. 3,785,723 issued Jan. 15, 1974 for "METHOD AND APPARATUS FOR LOCATING A PRINCIPLE MERIDIAN OF AN ASTIGMATIC OPTICAL SYSTEM", the disclosure of which is hereby incorporated by reference. In the '723 patent, a prior art technique is described, which employs a rotatable target in the form of an opaque disk with a series of small apertures arranged in a straight line along one diameter of the disk and illuminated from behind by a light source, thereby causing each of the apertures to appear as a point source of light. An imaging lens system having variable dioptric power is placed before the patient's eye for the purpose of correcting the refractive error of the eye or for aiding in the process of correcting the refractive error. If the eye is astigmatic, the image of each point aperture will appear to the patient as an image line segment. If the line drawn through the small apertures is aligned along the astigmatic axis (termed the "principle meridian" in the '723 patent), the image of the apertures will appear to the patient as a series of short line segments mutually aligned along a common axis. If the target is not so aligned with respect to the astigmatic axis of the patient's eye, the image will appear as a series of short line segments aligned in a mutually parallel fashion but laterally displaced from one another. By rotating the target until the short line segments are mutually aligned, the astigmatic axis can be determined with reference to a set of orthogonal reference coordinates. This information can then be used to align the laser apparatus to perform the cylindrical correction along the proper cylindrical axis. The '723 patent also discloses several embodiments of an improvement over the basic technique using a rotatable target, the improvements generally relating to various optical arrangements mechanically linked to the rotatable target for rotation therewith to provide better line segment image generation. It should be noted that the '723 patent teaches that the test image may be focused and aligned by the patient, by an examiner using auxiliary optical means to view the retina of the patient's eye, or by an auxiliary optical system having appropriate photoelectric detection means. Also, this patent states that the accuracy of the technique can be improved by using monochromatic or near-monochromatic light to form the test image.
While the '723 technique has been found useful for locating the astigmatic axis of a patient's eye, it does not provide in situ astigmatic axis alignment information just prior to ophthalmological surgery. In situ astigmatic axis alignment information for purposes of this invention is defined as such alignment information while the patient is in position for the corrective surgical technique and, more specifically, just prior to the start of the surgery. In situ astigmatic axis alignment is highly desirable since it provides real time information for adjustment of the cylinder axis of the laser surgery system and eliminates possible sources of alignment errors, such as rotation of the eye in the socket when the patient's horizontal reference is removed (e.g., when the patient is lying in a supine position in the operating chair) and rotation of the eye in the socket due to a shift from binocular to monocular viewing (e.g., when the patient views a target image with one eye through a monocular viewing instrument).